The present invention relates to a system for maintaining electrical continuity between adjacent conductive surfaces. In particular, the invention relates to a method and structure for preventing the leakage of electromagnetic waves and radio frequency waves through the junction of adjoining conductive panels wherein the panels are subject to relative movement.
Electronic equipment emits electromagnetic and radio frequency waves which may be detected by or interfere with other equipment. Such interference is commonly known as electromagnetic interface (EMI) and radio frequency interference (RFI). Methods have been developed for either isolating the source of the interference or for shielding other equipment from interfering waves. Ordinarily, equipment to be protected is located in a room or enclosure that is provided with EMI/RFI shielding. Alternatively, the equipment is encased in an EMI/RFI protective housing.
Full-sized rooms designed to shield EMI/RFI have typically been constructed from welded or bolted-together electrically conductive panels of sheet metal. Likewise, the protective housings for equipment have often commonly been formed from metal. The use of metal in this manner to protect equipment is expensive and cumbersome due to the cost and weight of the metal.
Methods have been developed for fabricating a lightweight, relatively inexpensive protective housing for equipment. For example, U.S. Pat. No. 4,678,699 to Kritchevsky et al. discloses a stampable thermoplastic composite having EMI/RFI shielding capabilities. The composite can be shaped into various configurations. The composite has at least one thermoplastic layer and one EMI/RFI shielding layer. The shielding layer is a fibrous or foraminous material such as nickel coated graphite fiber mat, metal fiber mat, knitted metal wire mesh or metal screen or, alternatively, a perforated metal sheet.
In another known method of fabrication, the protective housing is formed from plastic, and a layer of metal is arc-sprayed onto the plastic housing to provide the EMI/RFI protection. This concept of arc-spraying metal onto a surface to provide EMI/RFI shielding has been further extended to full sized rooms as discussed in Technical Report M-316 (U.S. Army Construction Engineering Research Laboratory, June 1982) entitled "Arc-Sprayed Metals for Structural Electromagnetic Shielding" and Technical Report M-332 (U.S. Army Construction Engineering Research Laboratory, August 1983) entitled "Electromagnetic Shielding of Full-Sized Structures By Metal Arc Spraying", both written by Paul Nielsen. These reports disclose studies related to metal arc-spraying of construction materials for EMI/RFI shielding.
A problem associated with both the metal arc-sprayed rooms and EMI/RFI protected rooms made from sheet metal, involves the leakage of electromagnetic and radio frequency waves. With arc-sprayed rooms, the metal lamina may crack due to settling or shifting of the building. The corners of the rooms are especially susceptible to cracking. Electromagnetic waves and radio frequency waves can also leak through the junctions of metal sheets used in forming a room or shelter.
Attempts have been made to provide shielding at the junction of structures where EMI/RFI leakage is likely to occur. U.S. Pat. No. 3,432,609 to W. C. Duvall et al. discloses a prefabricated shelter for shielding personnel and equipment from RFI. Inflatable panels, covered with a radio reflective material such as silver-coated nylon cloth, are interconnected to form the shelter. In order to provide radio reflective continuity between adjacent panels, a flexible metal screen is interposed between the panels so that the edges of the screen are in metal-to-metal contact with the radio reflective layer of the panels.
U.S. Pat. No. 4,655,012 to Downey et al. discloses a system for joining adjacent RFI protected buildings in a weather proof and RFI proof arrangement, wherein the buildings are subject to shifting relative to one another. A wire screen is shaped into a continuous loop much like a wire tube. The length of the wire tube is approximately equal to the distance between the buildings. In position, the ends of the wire tube are brought into electrical engagement with metallic skins already provided on the buildings as RFI shielding.
U.S. Pat. No. 4,115,656 to Aitel discloses a rigid-walled, telescoping passageway for interconnecting adjacent EMI shielded shelters where the shelters may not be in perfect alignment with respect to each other. The passageway comprises two aluminum overlapping sections. One section is telescopically received by the other. EMI shielding in the form of resilient conductive stuffing material or a rolling diaphragm is positioned between the overlapping portions of the passageway sections in order to maintain EMI shielding continuity between the sections. The rolling diaphragm is made from resilient layers of knitted metallic fiber with a resilient layer of electrically insulating material sandwiched between the metallic layers.